This invention relates to a process for producing medium carbon steel sheet and strip with a spheroidized microstructure having enhanced uniform elongation suitable for deep drawing applications. In one embodiment for producing the steel in hot rolled and annealed form, the steel is hot rolled with a lower than normal finishing temperature and subsequently annealed at a temperature below the A1 to provide an unexpected increase in uniform elongation. In a second embodiment for producing the steel in cold rolled and annealed form, increased lower yield strength levels are unexpectedly achieved at the higher end of the carbon range without significant decrease in uniform elongation by interaction of Mn and Si at various levels with the higher carbon.
There is a need for steel products having high strength and uniform elongation. High uniform elongation is particularly useful where the steel is to be formed into parts for automotive applications. In order to decrease the weight of cars and other vehicles, high strength along with high uniform elongation is particularly desired so that thinner gauge steels may be used. Dual phase steels developed for automotive applications typically have a proeutectoid ferrite microstructure with a significant fraction of low carbon martensite and/or lower bainite to provide high strength and high formability. Dual-phase steels generally require the addition of expensive alloying elements to increase hardenability and special cooling practices on a continuous annealing line or a hot strip mill to control the microstructure. Continuous annealing lines having controlled cooling capability are very expensive and require a substantial outlay of capital funds. There is a need therefore for steel producers that do not have continuous annealing lines with controlled cooling capability to be able to produce high strength steels having high uniform elongation using conventional batch or box annealing facilities.
A spheroidizing anneal below the A1 temperature is disclosed in U.S. Pat. No. 5,108,518 to Fukui et al. The reference discloses a method of manufacturing a steel sheet having high strength and excellent resistance to hydrogen embrittlement after heat treatments performed subsequent to the spheroidizing anneal. The steel is used for articles such as chain elements, gear members, clutch members, buckles for seat belts and washers. The steel consists essentially of C 0.30/0.70, Si 0.10/0.70, Mn 0.05/1.00, P not greater than 0.030, S not greater than 0.020, Cr 0.50/2.00, Mo 0.10/0.50, Ti 0.005/0.10, Nb 0.005/0.10, sol. Al not greater than 0.10, N greater than 0.002 but not greater than 0.015, optionally B 0.0005/0.002, balance iron and incidental impurities. The method includes hot rolling the steel with a finishing temperature of 800xc2x0 C. or higher, cooling at a rate of 5-40xc2x0 C./second to a temperature range of 500-700xc2x0 C. The hot rolled steel may optionally be cold rolled 20-80% and box annealed at a temperature of (Ac1xe2x88x9250) to (Ac1+30)xc2x0 C. for one hour or longer to spheroidize the cementites. Thin steel sheet produced by this method is formed and shaped by the customer and then subjected to heat treatment to provide sufficient hardness of the final product. The reference does not teach a hot rolled product that is spheroidize annealed, nor that lower hot roll finishing temperature will increase the uniform elongation of hot rolled steel strip after a spheroidizing anneal. The reference also does not teach the interaction of C, Mn, and Si for achieving various higher strength levels of cold rolled and spheroidize annealed strip product without substantial decrease in uniform elongation.
Several other references disclose spheroidization annealing of carbon steels below the A1 temperature. JP 11 264049 discloses a method of producing a high carbon steel strip free from shape defects such as sagging. The steel contains 0.2/0.8 C, up to 0.3 Si, 0.6/1.6 Mn, 0.01/0.1 sol. Al, 0.002/0.01 N, sol. Al/N: 5 to 10 and 0/0.01 Ca, the balance Fe with inevitable impurities. The steel is hot rolled, finish rolling at 850xc2x0 C. (from the example), coiled at 550/680xc2x0 C., cold rolled 10/80% annealed at 650/725xc2x0 C. and secondary cold rolled 5/25%. The spheroidizing ratio is regulated to up to 80%. Tensile strength (TS) is regulated between 600 to 700 N/mm2 and the TS (N/cm2)xc3x97(100xe2x88x92El %): 50xc3x97103 to 65xc3x97103. This reference does not disclose steel having high uniform elongation, nor controlling the hot roll finishing temperature to obtain high uniform elongation. The reference also does not disclose the interaction of C, Mn and Si on lower yield strength without significant decrease in uniform elongation after a spheroidizing anneal in cold rolled form. JP 57 134457 discloses improving the rate of spheroidization of the entire part of a hot rolled steel strip. The middle to high carbon steel is hot rolled, coiled and reheated after which it is again coiled. It is then mechanically descaled, pickled and spheroidization annealed. Applicants"" invention does not require reheating after hot rolling prior to a spheroidization anneal. JP 10 060540 discloses prevention of seizing flaws in steel strip. The steel is hot rolled to strip, pickled, descaled and repeatedly subjected to soaking and slow cooling below and above the A1 temperature. The spheroidizing rate improves and the production of a soft hot rolled steel strip is made possible. The strip is then cold rolled 20/85% and final annealed at a temperature between 630xc2x0 C. to the Ac1 temperature. This reference discloses additional reheating after hot rolling before cold rolling and spheroidize annealing which is unnecessary in Applicants"" invention. JP 61 076619 discloses a high carbon cold rolled steel strip having superior ductility. The steel contains 0.27/0.90 C, 0.15/0.30 Si, 0.60/0.90 Mn, up to 0.030 P, up to 0.035 S, and the balance Fe and inevitable impurities. The hot rolled strip is annealed for 15 hours at a temperature within the range 680/720xc2x0 C., cold rolled 20/45% and then annealed for 10 hours at a temperature within the same temperature range as the hot roll anneal. This reference requires a spheroidization anneal after both hot rolling and cold rolling which is not required in Applicants"" invention. Also this reference does not disclose hot rolling with a lower than normal finishing temperature to obtain enhanced uniform elongation. JP 31 22216 discloses a process for producing a cold rolled steel strip under the following conditions when the carbon content of the steel is less than 0.6%. The steel is hot rolled, coiled at a temperature within the range of 460-600xc2x0 C., cooled after hot rolling at a velocity regulated to 30-45xc2x0 C., cold rolled 50-85% and spheroidize annealed at a temperature between 680xc2x0 C. and the Ac3 temperature. This reference requires control cooling of the coil after hot rolling which is not required in Applicants"" invention. The reference also does not restrict the spheroidization temperature to a temperature below the A1 temperature.
A reference in which a sub-critical anneal is used to graphitize 50% or more of the cementite is disclosed in U.S. Pat. No. 5,454,887 to Fukui. The steel consists essentially of C 0.20/0.70, Si 0.20/2.00, Mn 0.05/0.50, P not more than 0.020, S not more than 0.010, sol. Al 0.01/1.00, B 0.0003/0.005, N 0.002/0.010, B/N 0.2/0.8, Cu 0/1.00, Ni 0/2.00, Ca 0/0.010, balance iron and incidental impurities. The steel is hot rolled with a finishing temperature of 700-900xc2x0 C., cooled at a rate of 5-50xc2x0 C./second and coiled at 400-650xc2x0 C. The steel is optionally cold rolled 20-85% and annealed at a temperature of 600xc2x0 C. to the Ac1 temperature for 1 hour or longer. The upper limit of 0.50% Mn is essential to ensure formation of graphite during the sub-critical anneal. To obtain graphitization, the chemical composition of the steel must be such that cementite is unstable at the temperature and time of the sub-critical anneal so as to permit breakdown of cementite into its constituent elements with the carbon going into its stable form as graphite. Applicants"" invention does not involve graphitization of the carbides in the steel. The reference does not teach the effect of lower finishing temperature on uniform elongation, or the interaction of C, Mn and Si on lower yield strength without decrease in uniform elongation.
A literature paper entitled xe2x80x9cSpheroidization of Medium-Carbon Steelsxe2x80x9d by J. M. O""Brien and W. F. Hosford, Journal of Materials Engineering and Performance, February, 1997, Vol. 6, pages 69-72, discloses spheroidization of AISI 4037 steel rod using both intercritical and subcritical annealing cycles to provide good cold formability of bolts made from the hot rolled rod. The paper discloses that bolts to be hardened by subsequent heat treatment are made from medium-carbon steel (0.35 to 0.50% C) alloyed for hardenablility with chromium, manganese and molybdenum. Rods of these steels are hot drawn to final diameter, coiled, and cooled for delivery. The structure at this point is ferrite and pearlite, with the coarseness of the pearlite depending on the rate of cooling after coiling. A spheroidization anneal is performed to provide sufficient formability for cold heading in which the head of the bolt is upset forged with a female die to form the bolt head. The reference does not teach or suggest that medium carbon steel sheet with high uniform elongation could be obtained by a spheroidization anneal below the A1 temperature, nor the effect of lower than normal finishing temperature on uniform elongation. The reference also does not suggest the interaction of C, Mn and Si on lower yield strength without decrease in uniform elongation in cold rolled and annealed steel strip.
A reference in which hot rolling of steel wire rod both above and below the A1 temperature is required in order to obtain a greater degree of spheroidization and softening is disclosed in U.S. Pat. No. 5,252,153 to Ochi et al. The steel contains 0.1 to 1.5% C, 0.25 to 2.0% Mn, balance iron and unavoidable impurities. Hot rolling is conducted at a temperature just above the Ar3 or just above the Arcm with a total reduction of area of 30% or more to form a pearlite having large lamellar spacing at the completion of transformation. The steel is further hot rolled at a temperature of from Ac1xe2x88x92400xc2x0 C. to the Ac1 with a total reduction of area of 10% to 70%. Spheroidization annealing is carried out by holding at a temperature of from 700 to 820xc2x0 C. for 2 to 7 hours and then gradually cooling the heated material to a temperature of from 600 to 720xc2x0 C. at a cooling rate of 0.1 to 1.0xc2x0 C./minute. Tensile strength after spheroidizing is reported lower and the degree of spheroidization is greater after hot rolling according to the process disclosed by the reference. The present invention does not involve hot rolling below the Ac1 temperature.
A high strength hot rolled steel sheet having excellent formability and spot weldability is disclosed in U.S. Pat. No. 5,505,796 to Kawano et al. In one embodiment the steel contains 0.15/less than 0.30 C, 0.5/3.0 Si, 0.5/3.0 Mn, more than 1.5 to 6.0 Si and Mn, not more than 0.02 P, no more than 0.01 S, and 0.005/0.10 Al, the balance essentially iron. The steel is hot rolled with a finish temperature in the range of Ar3xc2x150xc2x0 C. at an entire draft of not less than 80% and an ultimate strain speed of not less than 30/second. The steel is cooled on a hot runout table at a rate of not less than 30xc2x0 C./second followed by coiling at a temperature of more than 350 to 500xc2x0 C. The steel has a uniform elongation of not less than 10%. The reference does not disclose a spheroidizing anneal, nor the effect of lower than normal hot roll finishing temperature on uniform elongation after a spheroidizing anneal. The reference also does not disclose the interaction of C, Mn and Si on lower yield strength without significant decrease of uniform elongation in cold rolled and spheroidize annealed steel strip.
U.S. Pat. No. 4,021,272 to Asai et al discloses spheroidization annealing of a coil of hot rolled band steel for tools and razor blades by immersion of the coil with its convolutions spaced apart a minimum of 2 mm in a salt bath. The coil is immersed for 5 to 30 minutes and held at a temperature in a range of 550 to 750xc2x0 C. The reference does not disclose lower than normal hot roll finishing temperature, nor the interaction of C, Mn and Si on lower yield strength.
U.S. Pat. No. 5,516,373 to Dries et al discloses austempering of hot and cold rolled steel strapping containing 0.25/0.34 C, 1.20/1.55 Mn, up to about 0.035 Si, 0.201/0.45 V, or 0.35/0.45 Mo plus 0.12/0.18 V. The austempering step involves passing the strip through a first lead bath to preheat the strip to about 850xc2x0 F., resistance heating the strip to about 1600xc2x0 F., passing the strip through a second lead bath at about 800xc2x0 F. to quench the strip (and held at this temperature for about 8 seconds), and air-cooling to about 250xc2x0 F. followed by water cooling to room temperature. The resulting product has a non-equilibrium microstructure of very fine spheroidized carbides in ferrite. The reference does not disclose a sub-critical spheroidizing anneal, nor a steel suitable for deep drawing applications.
The present invention is of a method for producing medium-carbon steel sheet and strip having enhanced uniform elongation suitable for deep drawing applications. The product may be produced in either hot rolled and annealed or cold rolled and annealed form. In a first embodiment for producing the steel in hot rolled and annealed form, the method includes providing a steel slab having a composition consisting of in weight percent: 0.30/0.70 carbon, 0.75/2.0 manganese, up to 1.0 silicon, 0.020/0.10 total aluminum, the balance iron and incidental impurities. The slab is hot rolled to strip with a finish rolling temperature within the range of 839xc2x0 C. (1542xc2x0 F.) to 773xc2x0 C. (1424xc2x0 F. and then coiled. The hot rolled coil is box annealed at a temperature not greater than the A1 temperature, said temperature being within the range of A1 to 677xc2x0 C. (1250xc2x0 F.). The temperature and time at temperature during box annealing being effective to transform substantially all of the carbides in the microstructure of the steel to spheroidized form so that essentially none of the carbides are transformed to graphite, and to provide said hot rolled and box annealed strip with a minimum uniform elongation of at least about 15% and a lower yield strength within a range of about 50 ksi to about 60 ksi. Preferably the steel has a C content of 0.30/0.40.
In a second embodiment of the invention for producing the steel in cold rolled and annealed form, the method includes providing a steel slab having a composition consisting of in weight percent: 0.40 minimum/0.70 maximum carbon, 0.50/1.50 manganese, up to 1.0 silicon, 0.020/0.10 total aluminum, the balance iron and incidental impurities. The slab is hot rolled with a finish rolling temperature within the range of 900xc2x0 C. (1652xc2x0 F.) to the Ar1 temperature and then coiled. The hot rolled coil is cold rolled and box annealed at a temperature not greater than the A1 temperature, the temperature and time at temperature being effective to transform substantially all of the carbides in the microstructure to spheroidized form so that essentially none of the carbides are transformed to graphite and to provide the cold rolled and box annealed strip with a minimum lower yield strength of at least about 60 ksi and a minimum uniform elongation of at least about 14%. We have found that higher strength levels are achieved in cold rolled and annealed steels of this carbon range substantially without loss of uniform elongation by the interaction of manganese and silicon at various levels whereas the interaction effects are not found in the nominal 0.30%C cold rolled and annealed steels. The invention includes the steel products produced by the methods described above.